Statement: The quality of the pupillary light reflex as measured by portable pupillometers made by different companies is not the same.
Research Article
Two portable infrared pupillometers provide different measures to gauge the quality of the pupillary light reflex
https://doi.org/10.21203/rs.3.rs-4114696/v1
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Statement: The quality of the pupillary light reflex as measured by portable pupillometers made by different companies is not the same.
Pupil
Eye
Traumatic Brain Injury
Brain Stem
Coma
Heart Arrest
The quality of the pupillary light reflex as measured by portable pupillometers made by different companies is not the same.
The quality of the pupillary light reflex (PLR) is often used to evaluate midbrain function and third nerve compression following traumatic brain injury or cardiac arrest.1-3 Quantitative infrared pupillometry has been proposed to maximize the precision and inter-rater reliability of PLR assessment.4,5 Two commercial pupillometers measure multiple components of the PLR and provide an indexed measure, ranging from 0-5, indicating overall PLR reactivity (with any score < 3 considered reduced or abnormal). The NPI-200 pupillometer (Neuroptics, Irvine CA, USA) provides a value known as NPi, whereas the Neurolight (ID-Med, Marseille France) provides a value known as QPi. We compared the output of these two instruments after altering the size of the pupil, by consensual light stimulation to the unmeasured eye and by remifentanil administration. Of special interest was the quality of the light reflex after remifentanil because opioids have been shown to alter the reflex only by constricting the pupil.6 7 We hypothesized that the indexed readings would not be significantly altered by changes in the size of the pupil and that the values recorded from the two instruments would not differ significantly when sequential measurements were taken in the same subject under similar conditions.
After obtaining IRB approval, measures were made sequentially with each instrument in 10 healthy volunteer subjects. Informed consent was obtained from all individual participants included in the study. Five paid volunteers received remifentanil, with NPi and QPi taken every 2.5 minutes during 10-minute infusion and 25-minute recovery. A complete description of this type of study has been published.8 These prior studies confirm that the interventions described constrict the pupil to diameters that restrict the movement of the iris.9
Another five subjects were colleagues of the authors and had NPi and QPi taken as varying levels of light intensity were applied to the unmeasured eye (a dark closet, indoor ambient lighting, outdoor lighting, and flashlight illumination) to alter pupil size. Linear regression was performed modeling NPi and QPi against pupil size.
Subjects were studied between October 2022 and March 2023. Demographics were as follows: 48 ± 10 years old, 73 ± 8 kg; 6 were female. Pupil size decreased progressively with higher remifentanil concentration and greater consensual light stimulation. The Neurolight recorded a progressive decline in QPi as pupil size decreased, whereas the NeurOptics NPi readings remained unchanged (Figures 1 and 2). A strong relationship between QPi and pupil size was observed (coefficient = 0.78 [0.63, 0.92] aR2 = 0.53), whereas no significant relationship between NPi and pupil size was observed (coefficient = -0.04 [-0.09, 0.01], aR2 = 0.03). Joint hypothesis testing demonstrated a statistically significant difference in slope between the two models (F-statistic = 99, P < 0.0001). The relationship between the measures of light reflex quality did not differ between the two methods of altering pupil size (Figure 3).
We conclude that in this small sample of healthy volunteers, the two indexed measures indicating pupillary reactivity differed markedly between instruments when pupil size was altered by opioid or consensual light stimulation. The separation of the two responses was noted to be close to pupil sizes below 3.5 mm. This is the same diameter at or below which restriction of iris movement following a light stimulus has been observed.9-11 There are patients, especially the elderly, those with diabetes, and others receiving opioid medication, that may have constricted pupils without any abnormal or deteriorating neurologic deterioration. Thus, it is important to have a measure that evaluates the quality of the light reflex across the entire range of pupil diameters.
Although the NPi and the QPi are both promoted as objective indicators of the PLR, we conclude that these values are calculated differently. A full understanding of how the indexed values have been derived would be helpful when either device is used for clinical decision-making. Our model estimates an approximate 0.8-unit decrease in QPi with each 1-mm decrease below 3.5 mm pupil size, but no significant change in NPi across a range of pupil sizes.
A: Confirmation that manuscript complies with all instructions to authors: After reviewing the instructions to authors, I attest that the manuscript complies with the instructions to authors
B: Confirmation that authorship requirements have been met and the final manuscript was approved by all authors: The final manuscript was approved by both authors.
C: A list of each author’s contributions: Rachel Eshima McKay – helped to design the study, helped in the accumulation of the data, contributed to the writing of the manuscript. Merlin D. Larson – helped to design the study, helped to gather the data, wrote the initial draft of the manuscript, and approved the final document.
D: Confirmation that this manuscript has not been published elsewhere and is not under consideration by another journal: This manuscript has not been published elsewhere and is not under consideration by another journal.
E: Confirmation of adherence to ethical guidelines and indicate ethical approvals (IRB) and use of informed consent, as appropriate. Retrospective studies require a statement regarding IRB approval: The study was approved by the Institutional Review Board at the University of California, San Francisco.
F: Conflict of interest statement for all authors: Neither author has a conflict of interest. Also, neither author is a paid consultant to any commercial device manufacturers.
G: Confirmation of the use of a reporting checklist. If a checklist was used, please upload it with your submisstion. The Strobe checklist is enclosed with the submission.
Funding: Financial support was obtained from the Department of Anesthesiology and Perioperative Care, University of California, San Francisco and from a UCSF Mount Zion Health Fund, Grant # 20211335.
None of the Authors have potential conflicts of interest to be disclosed.
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